A digital signage system is an advertising medium which displays pictures and information on a display unit such as a flat display or a projector, utilizing digital technologies for display and communications. Digital signage systems are placed, for example, on walls in commercial facilities such as department stores and supermarkets or on sign boards or the like in public facilities such as railroads and roads. Generally, video content reproduction methods include a method of reproducing video data that has been compressed by an encoding technology conforming to a standard such as MPEG2 (Moving Picture Experts Group phase 2) or H.264, distributed via a network or the like, and saved temporarily, and another method of reproducing video data that is distributed in real time. Further, the digital signage system can adopt display methods such as a display method of arranging a plurality of display units to form a single large screen and another display method of arranging a plurality of display units vertically and horizontally and making the display units work together to display individual images.
For example, Patent Reference 1 proposes a method of synchronizing videos displayed by a plurality of display units. In this method, video is stored in a memory as video data, and image processors one-to-one corresponding to the plurality of display units read the video data from the memory while adjusting timing.
Patent Reference 2 proposes a method of synchronizing digital video signals individually produced by decoding in a plurality of reproduction units when displaying is performed on a screen by one-to-one connecting the plurality of reproduction units to display units. In this method, displaying is performed on the screen by using one of the plurality of reproduction units as a master reproduction unit and the other ones as slave reproduction units, sending a reference clock from the master reproduction unit to the slave reproduction units, and decoding data in the slave reproduction units in synchronization with the clock sent from the master reproduction unit.